1. Field
The presently disclosed subject matter relates to a filament electrode and to a fluorescent lamp using the same. More particularly, the disclosed subject matter relates to a filament electrode that can connect a filament coil with a pair of lead wires with confidence and can be employed even if an inner diameter of the light source is thin. The disclosed subject matter also relates to a fluorescent lamp using the above connection structures that can decrease both end areas which do not emit light and can allow emission with a wider range.
2. Description of the Related Art
A conventional hot-cathode fluorescent lamp (HCFL), cold-cathode fluorescent lamp (CCFL) and the like can include respective electrodes for supplying a power supply at both ends of a light tube that can include a glass tube, silica tube, quartz tube, or other type of arc/luminescence tube. The conventional filament electrode is composed of, for example, a filament coil that is connected to a pair of lead wires. The respective filament coils can be encapsulated in the light tube that includes a filler gas. Respective pairs of lead wires can extend from the light tube to an area outside of the light tube along and can be sealed within the tube structure in an air proof state. Therefore, when supplying the respective filament coils with a power supply via the respective pairs of lead wires, the conventional HCFL, CCFL and the like emit by generating a discharge in the light tube.
FIGS. 8A&B show a conventional structure of the above-described filament electrode, wherein FIG. 8(A) shows a state before fixing the lead wires 2a and 2b to a filament coil 3 via pressure bonding and FIG. 8(B) shows a state after fixing via pressure bonding. The filament electrode 1 is composed of both the filament coil 3 and the pair of lead wires 2a, 2b, which are made from respectively different materials. Therefore, the filament coil 3 and the pair of lead wires 2a, 2b should be fixed to each other. Each of the pair of lead wires 2a, 2b can be composed of a conductive metallic material and located parallel or substantially parallel to each other. The filament coil 3 is composed of, for instance, a tungsten, a doped tungsten and the like and includes a coil body 3a formed in a spiral and two connecting parts 3b, 3c jutted out from both ends of the coil body 3a in a straight line.
The filament coil 3 is attached to the pair of lead wires 2a, 2b as shown in FIGS. 8(A) and (B). That is to say, the filament coil 3 includes: each of the two connecting parts 3b, 3c thereof contacting each end of the pair of lead wires 2a, 2b perpendicular to each other; each end of the pair of lead wires 2a, 2b bending in a direction as shown by the arrows in FIG. 8A; each end of the pair of lead wires 2a, 2b sandwiching each of the two connecting parts 3b, 3c therebetween; and each end of the pair of lead wires 2a, 2b crimping each of the two connecting parts 3b, 3c therebetween. Thus, the filament coil 3 is attached to the pair of lead wires 2a, 2b by fixing the two connecting parts 3b, 3c thereof to each end of the pair of lead wires 2a, 2b with pressure bonding.
However, according to the conventional filament electrode 1 shown in FIGS. 8A&B, because the filament coil 3 extends in a direction towards an inner diameter of the light tube when manufacturing a HCFL and the like, it is difficult for a thin inner diameter type HCFL light tube and the like to employ the filament electrode 1.
Therefore, an electrode structure as shown in FIG. 9 is also well-known. The filament electrode 4 includes a pair of lead wires 2a, 2b located parallel to each other and attached to a filament coil 5 that is longer in a direction extending with the length of the pair of lead wires 2a, 2b (downwards in FIG. 9) than its interval. The above filament coil 5 is composed of a coil body 5a forming double helical so as to wrap in both directions of left and right around a central axis thereof and two connecting parts 5b, 5c extending symmetrically from both ends of the coil body 5a in parallel to the central axis, respectively.
The filament coil 5 includes: each of the two connecting parts 5b, 5c thereof contacting each end of the pair of lead wires 2a, 2b in line with each other; each of the two connecting parts 5b, 5c thereof being welded with each end of the pair of lead wires 2a, 2b using a spot welding, a laser welding and the like. Therefore, each of the two connecting parts 5b, 5c are fixed to each end of the pair of lead wires 2a, 2b. Thus, the filament coil 5 is attached to the pair of lead wires 2a, 2b by welding each of the two connecting parts 5b, 5c thereof with each end of the pair of lead wires 2a, 2b. In the above described electrode structure, heat generated from the filament coil 5 during light-emission is directly transmitted to the pair of lead wires 2a, 2b and is radiated.
A revised example of the above described filament electrode 4 is disclosed in, for example, Patent Document No. 1 (Japanese Patent Application Laid Open No. JP2005-235749). FIG. 10 is the disclosed electrode structure in accordance with Patent Document No. 1. Each of two connecting parts 5a, 5b of a filament coil 5 is weld to each of a pair of heat-tabs 6a, 6b and each end of a pair of lead wires 2a, 2b is also welded to each of the pair of heat-tabs 6a, 6b. That is to say, the filament coil 5 is attached to the pair of lead wires 2a, 2b via the pair of heat-tabs 6a, 6b with a weld. Therefore, the above electrode structure of FIG. 10 is configured to radiate heat generated from the filament coil 5 to the pair of lead wires 2a, 2b via the pair of heat-tabs 6a, 6b. 
FIG. 11 shows an electrode structure used for a bulb having double filaments. According to the electrode structure of the bulb 7 shown in FIG. 11, each of two filament coils 7a is welded to each of two lead wires 7b via each of two pipes 7c. Therefore, the electrode structure of the bulb 7 is configured to radiate heat generated from each of the two filament coils 7a to each of two lead wires 7b via each of the two pipes 7c. 
An exemplary electrode structure of a fluorescent lamp is disclosed in Patent Document No. 2 (Japanese Patent Application Laid Open No. Hei04-245161). FIG. 12 is the disclosed electrode structure of the fluorescent lamp in accordance with Patent Document No. 2. The fluorescent lamp 8 includes: a mixture 8c disposed in a metallic pipe 8b, the mixture 8c including a mixture of metallic powder with an emitter powder; the mixture 8c is pressed during manufacture so as to form a hole around a central axis thereof; the metallic pipe 8b is sintered to an electrode 8a located therein along with the mixture 8c; the metallic pipe 8b can then be tightened up at a back end thereof towards a central axis of lead wire 8e that passes through a end portion of glass tube 8d; thereby fixing the electrode 8a to the lead wire 8e. 
An exemplary electrode structure of a halogen bulb is disclosed in Patent Document No. 3 (Japanese Patent Application Laid Open No. Hei11-297272). FIG. 13 is the disclosed electrode structure of the halogen bulb 9. According to Patent Document No. 3, an electrode 9a is fixed to a lead wire 9b via a pipe 9c with a weld or a pressure bonding. In this case, because the pipe 9c is configured with a material of which the coefficient of thermal expansion (CTE) is bigger than that of tungsten which is the material of the electrode 9a, the pipe 9c expands because of heat generated from the electrode 9a during light-emission of the halogen bulb 9 and a gap is caused between the electrode 9a and the lead wire 9b. Thus, the heat generated from the electrode 9a cannot be directly transmitted to the lead wire 9b and an oxidation of the electrode 9a can be prevented.
The above-referenced Patent Documents are listed below and are incorporated herein by reference.
1. Patent Document No. 1: Japanese Patent Application Laid Open JP2005-235749
2. Patent Document No. 2: Japanese Patent Application Laid Open Hei04-245161
3. Patent Document No. 3: Japanese Patent Application Laid Open Hei11-297272
However, in the electrode structure of the filament electrode 4 shown in FIG. 9, the filament coil 5 is fixed to the pair of lead wires 2a, 2b by welding each of the two connecting parts 5b, 5c thereof with each end of the pair of lead wires 2a, 2b. Therefore, when welding each of the two connecting parts 5b, 5c with each end of the pair of lead wires 2a, 2b, a recrystallization of tungsten of the filament coil 5 can be caused, especially in the welding area. Therefore, the strength of fixing therebetween may become weak. As the result, the filament coil may become detached from the pair of lead wires 2a, 2b when a shock is applied thereto. A decrease of the fixing strength/intensity in a weld similar to the above-described weld can also be realized in the electrode structure of FIG. 10 and the bulb 7 of FIG. 11 described above.
In the above-described electrode structures of FIG. 8 to FIG. 10, heat generated from the filament coils 3, 5 is transmitted to the pair of lead wires 2a, 2b and is transmitted to a sealing portion between the pair of lead wires 2a, 2b and the glass tube. Thus, because a gap is caused in the sealing portion between the pair of lead wires 2a, 2b and the glass tube due to a difference between the CTE of glass of the sealing portion and that of a material of the pair of lead wires 2a, 2b, a filler gas in the glass tube may leak out. On the other hand, when a distance from each of the two connecting parts 3b, 3c and 5b, 5c of the filament coils 3, 5 to the sealing portion is extended by extending a length of the pair of lead wires 2a, 2b, the end areas of the tube that are not designed to emit light expands. Such an electrode structure is not desired for employment as a light source such as a back light unit and the like, because the both end areas in which light is not emitted is large.
In the fluorescent lamp 8 in accordance with Patent Document No. 2, the electrode 8a fixes the lead wire 8e by being sintered with the metallic pipe 8b and by fixing the back end of the metallic pipe 8b to the lead wire 8e with a pressure bonding. However, because the sinter bonding process for fixing the electrode 8a to the metallic pipe 8b is required, the process for manufacturing becomes complex.
In the electrode structure of the halogen bulb 9 in accordance with Patent Document No. 3, because the pipe 9c expands bigger than both the electrode 9a and the lead wire 9b during light-emission of the halogen bulb 9, a gap appears between the electrode 9a and the lead wire 9b. Thus, heat generated from the electrode 9a is not transmitted efficiently to the lead wire 9b because of the gap between the electrode 9a and the lead wire 9b, and the gap prevents the electrode 9a from oxidizing. However, because a transmission of the heat is shielded by the slight gap caused during light-emission of the halogen bulb 9 due to the difference of CTE between materials of both the electrode 9a and the lead wires 9c and a material of the pipe 9c, the heat-shield operation may be unstable or unpredictable, and electrical contact between the electrode 9a and the lead wire 9b may also become defective.
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, embodiments of the disclosed subject matter can include a filament electrode and associated stem that do not cause (or depreciates) some or all of the above-described various problems and can connect a filament coil with a pair of lead wires with confidence and strength. In addition, the filament electrode can be employed even if an inner diameter of a light source is very thin. The disclosed subject matter can also include a fluorescent lamp using the filament electrode that is configured to decrease that portion of the end areas that do not emit light, and to emit light with a wide range.